Many modern theories of morphology are highly modular: morphological phenomena with different properties are accounted for by separate modules of grammar. In this talk, I argue for a particular view of the modularity of morphology based on examination of Basque finite auxiliaries, which have a complex system of clitics that cross-reference ergative, absolutive and dative arguments in the sentence. I argue that a principled analysis of several generalizations regarding these clitics must involve a theory of word-formation with separate modules that have their own well-formedness principles and repair operations. Special attention will be given to verbal forms where these requirements of the separate modules conflict with each other. It will be shown that these conflicts are resolved in a manner that is best accounted for by a theory where the modules involved in word formation are derivationally ordered.